Final answer:
A guitar is designed to convert mechanical energy into sound energy, achieved through the vibration of its strings.
Step-by-step explanation:
The device designed to change mechanical energy to sound energy is a guitar. When a person plucks the strings of a guitar, it causes the strings to vibrate. These vibrations are mechanical energy, which the guitar then converts into sound energy that we can hear. This is an example of an energy transformation where mechanical energy is being transformed into acoustic (sound) energy.